‘大人買い (おとながい, otonagai)’ is an idiomatic phrase that literarily means ‘adult buying’, where ‘大人(おとな)’ means ‘adult’ and ‘買(が)い’ means ‘buying’. It is used when you purchase a large number of items at once; this is something that people used to yearn for in their childhood. In particular, it is often used when you splurge on the entire collection of items such as drama DVDs and books (manga). Since it is rather about the quantity than the price, kids with decent allowances can also do ‘adult buying’ (e.g., on snacks, card games).
I did 大人買い (‘adult buying’) on all volumes of ‘One Piece’(= Popular Japanese manga).
I did 大人買い (‘adult buying’) on a case of 24 cans of Coca Cola at the supermarket.
(Grammar Note) 「Noun + ごと」 = the whole thing including Noun
(e.g.) 箱ごと = the whole thing including the box
Synonyms: 奮発(ふんぱつ)する splurge, spend more money than usual, 箱買い （はこがい）buy a box of individual items)
Synonyms in English: splurge, buy a large amount of items
Related words and phrases: 大人(おとな) adult , 買う(かう) buy